The present invention relates to the field of digital signal processing, and in particular to a highly linear analog to digital conversion system with a tunable non-linear digital equalizer for providing an output signal with reduced non-linear distortions.
Digital signal processing is an important technology in many applications including communications, consumer electronics, radar, and sonar. In many of these applications there is a need to convert analog signals into digital form via an analog-to-digital converter (xe2x80x9cADCxe2x80x9d) with analog front-end devices such as amplifiers. These ADC convert continuous-time analog signals into discrete-time (i.e., sampled,) quantized digital signals.
In order to perform well, these ADCs and front-end devices are required to produce as little distortion as possible in the output signal. Distortions present in the ADC""s output signal can broadly be categorized as resulting from three sources:
a) Quantization distortion. Quantization distortion is the distortion caused by the ADC by allowing only a finite number of levels of output, including a maximum and a minimum output value. The quantization distortion has a non-linear characteristic that gives rise to signal distortions such as harmonics, intermodulation products, and signal correlated noise.
b) Deviations from the ideal response. Deviations from the ideal response can be caused by both the analog front-end devices and the ADC or the components singly or any combination thereof. Deviations from the ideal response include all distortions of the signal other than those caused by quantization. FIG. 3 graphically illustrates deviation between an ideal quantizer response and a non-ideal quantizer response.
c) Additive noise. Additive noise is caused mainly by the analog front-end devices.
While certain distortions, such as harmonics and signal-correlated noise are capable of being removed by linear filtering; however, intermodulation products, present particular difficulties, as intermodulation products are non-linear distortions that cannot be removed by linear filtering techniques.
There have been a number of attempts to reduce the distortions introduced by an ADC system. For instance, a technique known as dithering has been utilized to reduce quantization distortions. For dithering, noise is introduced to the input analog signal, thereby increasing the resolution of small values, allowing distortions caused by the quantization process to be reduced.
In addition, bounding the analog input signal reduces quantization distortions due to the finite dynamic range of the ADC. Other solutions are in the areas of more efficient design and manufacture of ADCs at the chip-level, which help to reduce additive noise and deviations from the ideal response.
Additional techniques such as digital-to-analog feedback and a phase space solution (e.g., using table lookup) have also been implemented. However, the digital-to-analog feedback only works with slow sampling rates, and it requires a digital-to-analog converter""s non-linear distortions to be less than the ADC""s non-linear distortions. The phase space technique depends strongly on ADC hardware and requires very large tables for ADC systems with memory longer than one tap. While these techniques help to alleviate some of the distortions introduced by ADC systems, there is still a need for better techniques of reducing non-linear distortions introduced into a signal by the analog-to-digital conversion process.
The present invention comprises an analog-to-digital converter (xe2x80x9cADCxe2x80x9d) and a tunable digital non-linear equalizer connected to the ADC, for reducing the non-linear distortions created by the ADC during an analog-to-digital signal conversion process. The equalizer implements a filtering function, which approximates the inverse of the non-linear distortion introduced into the signal by the ADC such that the overall non-linear distortion of the system is minimized under the constraints of the equalizer structure.
In another embodiment, at least one analog front-end device (e.g. amplifier or RF front-end) is connected to the ADC to perform processing on an analog signal before the signal is provided to ADC. The analog front-end device also creates non-linear distortions during the signal processing. Therefore, in this embodiment, the tunable digital non-linear equalizer is utilized to remove the non-linear distortions created by the analog front-end device, as well as, those created by the ADC.
Preferably, the equalizer is implemented as a Generate Function Streams unit, a plurality of linear finite impulse response (xe2x80x9cFIRxe2x80x9d) filters and a summer. The Generate Function Streams unit generates a plurality of outputs that are non-linear combinations of the ADC output, or, alternatively, combinations of the ADC output and equalizer output. Each of these streams is then passed through a corresponding FIR filter of the plurality of FIR filters. The outputs of the FIR filters are summed by a summer circuit to generate the output of the equalizer. Each of the FIR filters has tunable coefficients. An Identify Coefficients unit, preferably, adaptively sets IQ these coefficients. The coefficients set are identified by the Identify Coefficients unit using a test signal generator.
The test signal generator generates a predetermined functional stream for dynamically setting the FIR filter coefficients due to the change in the surrounding environment (such as thermal change). By supplying the predetermined functional stream to the Identify Coefficients unit, the equalizer coefficients of the plurality of FIR filters are determined through a matrix pseudo-inversion and normalization process.
The present invention further provides a method to compensate for the intermodulations introduced into the signal from the ADC that fall within a particular frequency band of interest. The digitized signal is first up-sampled by a factor of at least half the highest polynomial order of the highest order intermodulation to be removed, so as to prevent equalizer harmonics from being aliased into the frequency band of interest. This up-sampled signal is then shifted by at least half the bandwidth of interest. As a result of this, equalizer harmonics will not fall within the frequency band of interest. After the equalizer equalizes the shifted, up-sampled signal, the equalized signal is shifted back. Any out-of-band harmonics are then removed by linear filtering. Finally, the signal is down-sampled to its original sampling rate.
Other features and advantages of the present invention become apparent to one with ordinary skill in the art upon examination of the following drawings and detailed description. These additional features and advantages are intended to be included herein within the scope of the present invention.